Generally, light emitting diode (LED) is a kind of a semiconductor device, and it converts electricity into infrared ray or light by using a characteristic of a compound semiconductor, to send and receive a signal. The LED is used for home appliances, a remote controller, an electronic display board, a displaying apparatus, a variety of automation apparatuses and the like.
An operation principle of the LED will be briefly described in the following.
When a forward voltage is applied to a semiconductor of a specific chemical element, electrons and holes are recombined with each other while moving through a positive-negative junction. The recombination of the electrons and the holes causes an energy level to fall down, thereby emitting light.
Further, the LED is generally manufactured to have a very small size of 0.25 mm2 and is mounted on a lead frame or a printed circuit board (PCB) using has an epoxy mold.
Representative of the LEDs is a plastic package of 5 mm (T 1¾) or a new package being developed in a specific application field.
A color of light emitted from the LED is determined by a wavelength obtained depending on a combination of elements constituting a semiconductor chip.
Particularly, as an information communication apparatus is in a trend of a small size and slimness, the communication apparatus has more miniaturized parts such as a resistance, a condenser, and a noise filter. The LED is manufactured in a form of a Surface mounted Device (Hereinafter, referred to as “SMD”) so as to be directly mounted on a Printed Circuit Board (Hereinafter, referred to as “PCB”).
Accordingly, an LED lamp for a display device is being developed in the form of the SMD. Such an SMD can substitute a related-art simple lamp. The SMD is used for a lamp display, a character display, an image display and the like that express various colors.
Further, as a high-density integration technology for a semiconductor device is developed and a consumer prefers a more compact electronic product, Semiconductor Mounting Technology (SMT) is widely used, and a packaging technology of the semiconductor device employs a technology for minimizing an installation space such as a Ball Grid Array (BGA), a wire bonding, and a flip chip bonding.
FIG. 1 is a view illustrating a related art light emitting diode.
As shown in FIG. 1, after an N-type nitride gallium layer 102 is formed on a sapphire substrate 101, an N-type electrode 105 is formed at one portion of and on the N-type nitride gallium layer 102. A metal organic chemical vapor deposition (MOCVD) is used to form a film using a Group 3-based element on the sapphire substrate 101.
Silicon using silane (SiH4) gas is used to form N-type dopants. All three-component nitride films are grown in an atmosphere of Hydrogen gas. Nitrogen gas is used to grow a nitride gallium.
After the N-type nitride gallium 102 is formed, an active layer 104 is formed on the N-type nitride gallium layer 102. The active layer 104 corresponding to a light emission region has a quantum well structure comprising a Indium gallium nitride. After the active layer 104 is formed, a P-type nitride gallium (GaN) layer 106 is formed.
The P-type nitride gallium layer 106, which is contrasted with the N-type nitride gallium layer 102, is formed by the addition of P-type dopants. Therefore, in the N-type nitride gallium layer 102, electrons are drifted by an external voltage. In the P-type nitride gallium layer 106, holes are drifted by the external voltage. The electrons and the holes are recombined with each other to emit light.
The transparent metal-based transparent electrode 107 is formed on the P-type nitride gallium layer 106 to allow light generated from the active layer 104 to be transmitted therethrough, and allow the generated light to be emitted to the external.
After the transparent electrode 107 is formed, a P-type electrode 103 is formed to compose the light emitting diode.
However, since the related-art light emitting diode using the sapphire substrate has a nitride film having a larger refractive index than the substrate, the light generated from the active layer is transmitted through the nitride film to be emitted toward the transparent electrode.
FIG. 2 is a plan view illustrating the transparent electrode, the P-type electrode, and the N-type electrode of the light emitting diode of FIG. 1.
As shown in FIG. 2, the P-type electrode 103 and the N-type electrode 105 are disposed at both portions of the light emitting diode 100, and the transparent electrode 107 is disposed on a whole region of the light emitting diode 100.
The transparent electrode 107 corresponds to a region at which the light generated from the active layer of the light emitting diode 100 is emitted. The transparent electrode 107 is formed of a transparent conductive metal.
FIGS. 3a and 3b are views illustrating a light emission region of a related art light emitting diode. As shown, it can be appreciated that the generated light is biased and concentrated at a predetermined region.
In other words, a light biased-concentration phenomenon occurs since a voltage applied to combine the holes and the electrons in the active layer is varied due to a resistance component centering on the N-type electrode. As a distance is increased from the N-type electrode, resistance is increased. Therefore, the increased resistance again results in a voltage drop.
Unlike this, the resistance is not varied depending on the distance at a region far away from the N-type electrode comparing to a region close to the N-type electrode. Therefore, a high current is applied to the active layer when a high voltage is applied to an electrode region.
If the high voltage is applied to the region close to the electrode region as described above, a temperature of the active layer rises. The rise of the temperature of the active layer causes a drop of a turn-on voltage of the light emitting diode to make worse the biased-concentration of a driving current.
Further, as shown in FIG. 3b, in case where the N-type electrode is disposed along a circumference of an upper edge of the light emitting diode, the light is biased and concentrated only at an N-type electrode region, which is close to the P-type electrode.
Specifically, the P-type electrode itself functions to block or absorb a portion of photons incident on the P-type electrode, thereby deteriorating an efficiency of light emission. If a thickness of the P-type electrode is decreased to prevent this, the resistance of the P-type electrode is relatively greatly increased. Accordingly, it cannot be expected that the light is emitted from the region far away from the n-type electrode.
The light biased-concentration phenomenon occurs because the voltage applied to the active layer is not constantly maintained at the whole region along the P-type electrode and the transparent electrode, due to the resistance increased at each distance away from the electrode and different voltage drops caused by the increased resistance at each upper electrode region.
Further, in case where the emitted light is reflected or absorbed by the P-type electrode, the light efficiency is deteriorated. Specifically, the light is absorbed at the transparent electrode formed of the transparent metal, an emitted amount of light is reduced.
Additionally, since most of the generated light of the light emitting diode is emitted through the transparent electrode to the external, the reduction of the light efficiency causes the reduction of an external quantum efficiency.